1. Field of the Invention
This invention relates to the field of image projection systems and more particularly to phase contrast image projection systems.
2. Background of the Invention
Spatial light modulators (SLM) are used to modulate radiant energy, such as visible and non-visible light, in optical systems. Spatial light modulators are divided into one or two dimensional arrays of modulation elements called pixels, or picture elements, that represent the smallest addressable unit within the device. The SLM pixel is used to modulate the light distribution within the optical system.
In practice, the light modulation characteristics of most prior art SLMs are coupled combinations of amplitude and phase changes. The modulation characteristic of a pixel is usually controlled by a single applied signal, either an electrical voltage, current or incident optical intensity level, so the amplitude and phase characteristics of the pixel cannot be independently
Depending on the design of the SLM, either the amplitude or phase modulation characteristic may dominate the output of the SLM. One type of SLM is the digital micro-mirror device (DMD). DMDs have been used to project images using either Schlieren or dark-field optics. Ott teaches the use of a membrane DMD and a cantilever beam DMD with Schlieren optics to amplitude modulate light in U.S. Pat. No. 4,680,579 issued Jul. 14, 1987. Hornbeck teaches the use of a digital torsion beam DMD and a cantilever beam DMD with dark-field optics to amplitude modulate light in U.S. Pat. No. 5,096,279 issued Mar. 17, 1992. Florence teaches the use of a flexure beam DMD to either phase modulate, or both amplitude and phase modulate light in U.S. Pat. No. 5,148,157 issued Sep. 15, 1992. To independently modulate both the amplitude and phase of incident light requires two SLM elements which requires either doubling the number of elements in the array or reducing the image resolution. This constraint, especially for high resolution display systems, increases both the manufacturing cost of the SLM and the complexity of the associated drive electronics.
The human eye, as well as most image sensors, does not detect phase changes in incident light. Therefore, in order to project a visible image, systems have been limited to simple techniques creating an image using amplitude modulation techniques with associated dependence on Schlieren optics or digital torsion beam implementations with dark field optics. Schlieren optical systems generally have low brightness and require a large light source. Digital torsion beam approaches turn a pixel either on or off and require some type of pulse-width modulation to achieve varying contrast or gray scale output levels. Pulse-width modulation can create visual artifacts in displayed images and typically requires image sensors to be synchronized to prevent image flicker.
A need exists for a modulation scheme that can produce bright, gray scale visual images while eliminating flicker and reducing or eliminating visual artifacts.